Type 1 diabetes (T1D) is an autoimmune disease triggered by T cell reactivity to protein antigens produced by the -cells. Here we present a chronological compendium of transcriptional profiles from islets of Langerhans isolated from non-obese diabetic (NOD) mice ranging from 2 wks up to diabetes and compared to controls. Parallel analysis was made of cellular components of the islets. Myeloid cells populated the islets early during development in all mouse strains. This was followed by a type I interferon signature detectable at 4-6 wks of age only in diabetes susceptible mice. Concurrently, CD4 T cells were found within islets, many in contact with intra-islet antigen presenting cells. Early cellular signs of islet reactivity were detected by six wks. By 8 wks, NOD islets contained all major leukocytes populations and an inflammatory gene signature. This work establishes the natural transcriptional signature of T1D and provides a resource for future research.